Earth  Planets and Moons 

Moon Phase and Libration, 2011

Dial-A-Moon

Month: Day: UT Hour:



The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2011, at hourly intervals. Until the end of 2011, the initial Dial-A-Moon image will be the frame from this animation for the current hour.

More in this series: 2015 | 2015 South | 2014 | 2014 South | 2013 | 2013 South | 2012

This marks the first time that accurate shadows at this level of detail are possible in such a computer simulation. The shadows are based on the global elevation map being developed from measurements by the Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO). LOLA has already taken more than 10 times as many elevation measurements as all previous missions combined.

The Moon always keeps the same face to us, but not exactly the same face. Because of the tilt and shape of its orbit, we see the Moon from slightly different angles over the course of a month. When a month is compressed into 12 seconds, as it is in this animation, our changing view of the Moon makes it look like it's wobbling. This wobble is called libration.

The word comes from the Latin for "balance scale" (as does the name of the zodiac constellation Libra) and refers to the way such a scale tips up and down on alternating sides. The sub-Earth point gives the amount of libration in longitude and latitude. The sub-Earth point is also the apparent center of the Moon's disk and the location on the Moon where the Earth is directly overhead.

The Moon is subject to other motions as well. It appears to roll back and forth around the sub-Earth point. The roll angle is given by the position angle of the axis, which is the angle of the Moon's north pole relative to celestial north. The Moon also approaches and recedes from us, appearing to grow and shrink. The two extremes, called perigee (near) and apogee (far), differ by more than 10%.

The most noticed monthly variation in the Moon's appearance is the cycle of phases, caused by the changing angle of the Sun as the Moon orbits the Earth. The cycle begins with the waxing (growing) crescent Moon visible in the west just after sunset. By first quarter, the Moon is high in the sky at sunset and sets around midnight. The full Moon rises at sunset and is high in the sky at midnight. The third quarter Moon is often surprisingly conspicuous in the daylit western sky long after sunrise.

Celestial north is up in these images, corresponding to the view from the northern hemisphere. The descriptions of the print resolution stills also assume a northern hemisphere orientation. To adjust for southern hemisphere views, rotate the images 180 degrees, and substitute "north" for "south" in the descriptions.


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Visualization Credits

Ernie Wright (UMBC): Lead Animator
Marte Newcombe (GST): Animator
Chris Smith (HTSI): Producer
Richard Vondrak (NASA/GSFC): Scientist
Joycelyn Thomson Jones (NASA/GSFC): Project Support
Please give credit for this item to:
NASA/Goddard Space Flight Center Scientific Visualization Studio

Short URL to share this page:
http://svs.gsfc.nasa.gov/goto?3810

Missions:
Clementine
LRO (Lunar Reconnaissance Orbiter)

Data Used:
LRO/LOLA/Digital Elevation Map August 2009 - September 2010
Clementine/UVVIS Camera/750-nm Basemap 1994-02-26 to 1994-04-21

This item is part of these series:
The Moon
LRO - Animations

Keywords:
SVS >> Elevation data
SVS >> HDTV
SVS >> Laser Altimeter
SVS >> Lunar
SVS >> Moon
SVS >> LRO
SVS >> Lunar Reconnaissance Orbiter
SVS >> LOLA
NASA Science >> Earth
NASA Science >> Planets and Moons